In case you were wondering...

Here's a nice little breakdown of how things work in this city. Complicated for nothing, I think. It's no wonder things are falling apart. 

Basically, every borough needs a minimum of 5 people sitting in the
borough council. Second to that, every borough has the right to one
representative at the city council per roughly 15000 registered voters.
Mayors and city councillors have the right to sit at city council.
Borough councillors only have the right to sit at the borough council.

There are three types of compositions.
1. Boroughs with a mayor and only city councillors
2. Boroughs with a mayor and only borough councillors
3. Boroughs with a mayor and a mix of city and borough councillors.

Here are some examples. In Villeray, with 86000 registered voters, you
have the right to 5 people sitting at city council. Thus, the mayor of
the borough and 4 city councillors. They act as local representatives
and representatives at city council. All the big ones follow this
pattern (CDN-NDG, Rosemont, etc.)

Take Outremont, with 15000 registered voters. They can only have one
person sitting at city council. That would be the mayor. But they need a
minimum of 5 people sitting at the borough council. So the other 4 are
necessarily borough councillors. All the small boroughs follow this
pattern (like Ile Bizard)

Finally, Montreal North with 52000 reigstered voters, thus they have the
right to 3 people sitting at city council: the mayor and 2 city
councillors. They also need 5 people sitting in the borough council. So
the final 2 people are borough councillors. The mid-size boroughs follow
this pattern.

The make up of each borough is explained on this site

http://election-montreal.qc.ca/userfiles/file/fr/cadre-electoral/DistrictsElectoraux2013/Cadre_Electoral_2013-2017_Synth%C3%A8se_20130517.pdf

Essentially, the wide disparity between borough populations explains
this rather complex arrangement.

Bon vote.